This invention relates generally to a nozzle for use in a gas burner and more specifically to a nozzle which is particularly suitable for use in a high excess air burner that operates over a relatively wide range of fuel flow rates.
A nozzle of this general type is constructed with multiple combustion chambers. Each combustion chamber has an inlet opening for receiving the fuel-air mixture from the previous or upstream chamber and typically has additional passages for receiving additional combustion air. Combustion air is staged through the axially spaced passages to allow the nozzle to operate over the range of fuel flow rates. For example, as the flow of fuel increases and the fuel-air mixture becomes fuel rich in one combustion chamber, the flame front moves forwardly or downstream into the next combustion chamber where additional air is delivered to the flame through additional openings. The additional air brings the volumetric fuel-to-air ratio of the mixture in the downstream combustion chamber to within the flammability limits of the fuel. Alternately, as the flow of fuel decreases and the fuel-air mixture becomes fuel lean in one combustion chamber, the flame front moves upstream into the previous combustion chamber where one less set of openings delivers air to the mixture. In a similar manner, if the flow of air delivered to the flame increases to a point where the fuel-air mixture becomes fuel lean in one combustion chamber, the flame front transitions upstream into the previous combustion chamber. During normal operation of a high excess air burner, the flame front transitions throughout the nozzle as the volumetric flow rate of either the air or the fuel varies within the predefined operating limits of the burner.
To provide for stable flame retention in each combustion chamber, the combustion chamber is constructed so that the fuel-air mixture expands as it enters that combustion chamber. The relatively high velocity of the fuel-air mixture at the inlet opening, as compared to the velocity of the expanded fuel in the upstream end of the chamber, prevents the flame from flashing back toward the source of the fuel.
Successful operation of a nozzle having more than one combustion chamber requires that the flame smoothly transition between adjacent combustion chambers. In the absence of provisions for a smooth flame transition, there tends to be a region of unstable operation where the flame is unable to establish itself in either of the adjacent combustion chambers. This instability is substantially due to the turbulent nature of the fuel-air mixture as it flows through the inlet opening of the downstream combustion chamber and tends to cause the flame front to jump back and forth between the two adjacent combustion chambers. Under extremely turbulent conditions at high flow rates, this instability may cause the flame to be extinguished.
If the upstream chamber is operating at a relatively low fuel flow rate, a smooth flame transition may be provided by causing the fuel-air mixture in the upstream chamber to swirl as it exits that chamber and enters the next chamber. However, if the fuel flow rate is relatively high, a swirling fuel-air mixture detrimentally affects the operating efficiency of the nozzle.